1. Universe Tenth Edition
Roger Freedman • Robert Geller • William Kaufmann III
Universe Tenth Edition
Chapter 8
Comparative Planetology II: The Origin of Our Solar System

2. Chapter 8 Opener: Planets are thought to form within the disks surrounding young stars such as these. Neptune’s orbit, shown for scale, is about 60 AU across. (NASA, ESA, D. R. Ardila (JHU), D. A. Golimowski (JHU), J. E. Krist (STScI/JPL), M. Clampin (NASA/GSFC), J. P. Williams (UH/IfA), J. P. Blakeslee (JHU), H. C. Ford (JHU), G. F. Hartig (STScI), G. D. Illingworth (UCO-Lick) and the ACS Science Team)

3. By reading this chapter, you will learn
8-1 The key characteristics of the solar system that must be explained by any theory of its origins
8-2 How the abundances of chemical elements in the solar system and beyond explain the sizes of the planets
8-3 How we can determine the age of the solar system by measuring abundances of radioactive elements
8-4 Why scientists think the Sun and planets all formed from a cloud called the solar nebula
8-5 How the solar nebula model explains the formation of the terrestrial planets
8-6 How the Jovian planets formed and migrated in the early solar system
8-7 How astronomers search for planets around other stars

4. 8-1: Any model of solar system origins must explain the present-day Sun and planets
Table 8-1

5. Composition of the Solar System
Figure 8-1: Hydrogen and helium make up almost all of the mass of our solar system. Other elements such as carbon, oxygen, nitrogen, iron, gold, and uranium constitute only 2% of the total mass.
Composition of the Solar System

6. A Massive Star Ejecting Gas and Dust
Figure 8-2: The star Antares is shedding material from its outer layers, forming a thin cloud around the star. We can see the cloud because some of the ejected material has condensed into tiny grains of dust that reflect the star’s light. (Dust particles in the air around you reflect light in the same way, which is why you can see them within a shaft of sunlight in a darkened room). Antares lies some 600 light-years from Earth in the constellation Scorpio. (David Malin/Anglo-Australian Observatory)
A Massive Star Ejecting Gas and Dust

7. A Massive Star Ejecting Gas and Dust
Figure 8-2: The star Antares is shedding material from its outer layers, forming a thin cloud around the star. We can see the cloud because some of the ejected material has condensed into tiny grains of dust that reflect the star’s light. (Dust particles in the air around you reflect light in the same way, which is why you can see them within a shaft of sunlight in a darkened room). Antares lies some 600 light-years from Earth in the constellation Scorpio. (David Malin/Anglo-Australian Observatory)
A Massive Star Ejecting Gas and Dust

8. New Stars Forming From Gas and Dust
Figure 8-3: Unlike Figure 8-2, which depicts an old star that is ejecting material into space, this image shows young stars in the constellation Orion (the Hunter) that have only recently formed from a cloud of gas and dust. The bluish, wispy appearance of the cloud (called NGC ) is caused by starlight reflecting off interstellar dust grains within the cloud (see Box 5-4). The grains are made of heavy elements produced by earlier generations of stars. (David Malin/Anglo-Australian Observatory)
New Stars Forming From Gas and Dust

9. New Stars Forming From Gas and Dust
Figure 8-3: Unlike Figure 8-2, which depicts an old star that is ejecting material into space, this image shows young stars in the constellation Orion (the Hunter) that have only recently formed from a cloud of gas and dust. The bluish, wispy appearance of the cloud (called NGC ) is caused by starlight reflecting off interstellar dust grains within the cloud (see Box 5-4). The grains are made of heavy elements produced by earlier generations of stars. (David Malin/Anglo-Australian Observatory)
New Stars Forming From Gas and Dust

10. 8-2: The cosmic abundances of the chemical elements are the result of how stars evolve
Figure 8-4: This graph shows the abundances in our part of the Galaxy of the 30 lightest elements (listed in order of increasing atomic number) compared to a value of 1012 for hydrogen. The inset lists the 10 most abundant of these elements, which are also indicated in the graph. Notice that the vertical scale is not linear; each division on the scale corresponds to a tenfold increase in abundance. All elements heavier than zinc (Zn) have abundances of fewer than 1000 atoms per 1012 atoms of hydrogen.
Abundances of the Lighter Elements

11. 8-2: The abundances of radioactive elements reveal the solar system’s age
Figure 8-5: Although it resembles an ordinary Earth rock, this is actually a meteorite that fell from space. The proof of its extraterrestrial origin is the meteorite’s composition and its surface. Searing heat melted the surface as the rock slammed into our atmosphere. Meteorites are the oldest objects in the solar system. (Ted Kinsman/Photo Researchers, Inc.)
A Meteorite

12. Box 8-1 Tools of the Astronomer’s Trade: How old are the rocks found on Earth and other planets? Are rocks found at different locations the same age or different ages? How old are meteorites? Questions like these are important to scientists who wish to reconstruct the history of our solar system. The age of a rock is how long ago it solidified, but simply looking at a rock cannot tell us whether it was formed a million years, or a billion years ago. Fortunately, most rocks contain trace amounts of radioactive elements such as uranium. By measuring the relative abundances of various radioactive isotopes and their decay products within a rock, scientists can determine the rock’s age.
Radioactive Dating

13. 8-2: The Sun and planets formed from a solar nebula
Figure 8-6: (a) A cloud of interstellar gas and dust begins to contract because of its own gravity. (b) As the cloud flattens and spins more rapidly around its rotation axis, a central condensation develops that evolves into a glowing protosun. The planets will form out of the surrounding disk of gas and dust.
The Birth of the Solar System

14. Figure 8-6a: A cloud of interstellar gas and dust begins to contract because of its own gravity.

15. Figure 8-6b: As the cloud flattens and spins more rapidly around its rotation axis, a central condensation develops that evolves into a glowing protosun. The planets will form out of the surrounding disk of gas and dust.

16. Conservation of Angular Momentum
Figure 8-7: A figure skater who (a) spins slowly with her limbs extended will naturally speed up when (b) she pulls her limbs in. In the same way, the solar nebula spun more rapidly as its material contracted toward the center of the nebula. (AP Photo/Amy Sancetta)
Conservation of Angular Momentum

17. Figure 8-8: (a) The Orion Nebula is a star-forming region located some 1500 light-years from Earth. It is the middle “star” in Orion’s “sword” (see Figure 2-2a). The smaller, bluish nebula is the object shown in Figure 8-3. (b) This view of the center of the Orion Nebula is a mosaic of Hubble Space Telescope images. The four insets are false-color close-ups of four protoplanetary disks that lie within the nebula. A young, recently formed star is at the center of each disk. (The disk at upper right is seen nearly edge-on.) The inset at the lower left shows the size of our own solar system for comparison. (a: Anglo-Australian Observatory image by David Malin; b: C. R. O’Dell and S. K. Wong, Rice University; NASA)
Protoplanetary Disks

18. Figure 8-9: This highly magnified image shows a microscopic dust grain that came from interplanetary space. It entered Earth’s upper atmosphere and was collected by a high-flying aircraft. Dust grains of this sort are abundant in star-forming regions like that shown in Figure 8-3. These tiny grains were also abundant in the solar nebula and served as the building blocks of the planets. (NASA)
A Grain of Cosmic Dust

19. Temperature Distribution in the Solar Nebula
Figure 8-10a: This graph shows how temperatures probably varied across the solar nebula as the planets were forming, and the present-day position of the planets (red arrows). Note the general decline in temperature with increasing distance from the center of the nebula. Between the present-day distances of Mars and Jupiter, the snow line marked where temperatures were low enough for water to condense and form ice; beyond about 16 AU, methane (CH4) could also condense into ice.
Temperature Distribution in the Solar Nebula

20. Snow Line in the Solar Nebula
Figure 8-10b: Terrestrial planets formed inside the snow line, where the low abundance of solid dust grains kept these planets small. The Jovian planets formed beyond the snow line where solid ices of water, methane, and ammonia added their mass to build larger cores, and attract surrounding gas.
Snow Line in the Solar Nebula

21. 8-5: The terrestrial planets formed by the accretion of planetesimals
Figure 8-11: This computer simulation shows the formation of the inner planets over time. (Adapted from George W. Wetherill)
Accretion of the Inner Planets

22. Accretion of the Inner Planets
Figure 8-11: This computer simulation shows the formation of the inner planets over time. (Adapted from George W. Wetherill)
Accretion of the Inner Planets

23. Figure 8-12: This is a cross-section from a fragment of the Allende meteorite that landed in Chihuahua, Mexico, in A fireball was observed at 1:05 a.m. local time, and over the next 25 years about 3 tons of meteorite fragments were collected over an 8 km by 50 km area. Radioactive dating yields an age of 4.57 billion years, corresponding to the very early solar system. The meteorite contains rocks and metals that are consistent with the inner solar system and is thought to come from a smashed-up planetesimal. (ChinellatoPhoto/Shutterstock)
Primitive Meteorite

24. The Formation of the Solar System
Figure 8-13 part 1: This sequence of drawings shows stages in the formation of the solar system.
The Formation of the Solar System

25. 8-5: Gases in the outer solar nebula formed the Jovian planets and planetary migration reshaped the solar system
Figure 8-13 part 1: This sequence of drawings shows stages in the formation of the solar system.
The Formation of the Solar System

26. The Formation of the Solar System
Figure 8-13 part 2: This sequence of drawings shows stages in the formation of the solar system.
The Formation of the Solar System

27. The Formation of the Solar System
Figure 8-13 part 2: This sequence of drawings shows stages in the formation of the solar system.
The Formation of the Solar System

28. The Formation of the Solar System
Figure 8-13: This sequence of drawings shows stages in the formation of the solar system.
The Formation of the Solar System

29. The Formation of the Solar System
Figure 8-13: This sequence of drawings shows stages in the formation of the solar system.
The Formation of the Solar System

30. The Kuiper Belt and Oort Cloud
Figure 8-14a part 1: The classical Kuiper belt of comets spreads from Neptune out to 50 AU from the Sun. Most of the estimated 200 million belt comets are believed to orbit in or near the plane of the ecliptic. The spherical Oort cloud extends from beyond the Kuiper belt.
The Kuiper Belt and Oort Cloud

31. The Kuiper Belt and Oort Cloud
Figure 8-14a part 2: The classical Kuiper belt of comets spreads from Neptune out to 50 AU from the Sun. Most of the estimated 200 million belt comets are believed to orbit in or near the plane of the ecliptic. The spherical Oort cloud extends from beyond the Kuiper belt.
The Kuiper Belt and Oort Cloud

32. Figure 8-14b: Orbits of bodies in the Kuiper belt. (NASA and A
Figure 8-14b: Orbits of bodies in the Kuiper belt. (NASA and A. Field/Space Telescope Science Institute)

33. Imaging Extrasolar Planets
Figure 8-15: (a) Four planets can be seen orbiting the star HR The star is about 129 light-years from Earth and can be seen with the naked eye (RVIUXG) on a clear night. In capturing this image, special techniques make the star less visible to reduce its glare and reveal its planets. The white arrows indicate possible trajectories that each star might take over the next ten years. (b) About 170 light-years away, the star 2M1207 and a planet with about 1.5 times the diameter of Jupiter are captured in this infrared image. First observed in 2004, this extrasolar planet was the first to be visible in a telescopic image. At infrared wavelengths a star outshines a Jupiter-sized planet by only about 100 to 1, compared to 109 to 1 at visible wavelengths, making imaging possible in the infrared. (a: NASA/W. M. Keck Observatory; b: ESO/VLT/NACO)
Imaging Extrasolar Planets

34. 8-7: A variety of observational techniques reveal planets around other stars
Figure 8-16: (a) A planet and its star both orbit around their common center of mass, always staying on opposite sides of this point. Even if the planet cannot be seen, its presence can be inferred if the star’s motion can be detected. (b) The astrometric method of detecting the unseen planet involves making direct measurements of the star’s orbital motion. (c) In the radial velocity method, astronomers measure the Doppler shift of the star’s spectrum as it moves alternately toward and away from Earth. Analyzing the Doppler shift of the star’s light leads to the orbital period, orbital size (semimajor axis), and mass of the unseen planet. The radial velocity method works for circular and elliptical orbits and can even determine the orbital eccentricity.
Detecting a Planet by Measuring Its Parent Star’s Motion

35. Figure 8-16a: A planet and its star both orbit around their common center of mass, always staying on opposite sides of this point. Even if the planet cannot be seen, its presence can be inferred if the star’s motion can be detected.

36. Figure 8-16b: The astrometric method of detecting the unseen planet involves making direct measurements of the star’s orbital motion.

37. Figure 8-16c: In the radial velocity method, astronomers measure the Doppler shift of the star’s spectrum as it moves alternately toward and away from Earth. Analyzing the Doppler shift of the star’s light leads to the orbital period, orbital size (semimajor axis), and mass of the unseen planet. The radial velocity method works for circular and elliptical orbits and can even determine the orbital eccentricity.

38. Discovering Planets from a Star’s Wobble: 51 Pegasi
Figure 8-17: As a star wobbles, it moves toward us and away from us, with a varying radial velocity that is measured through the Doppler shift. The shape of the curve indicates the planet’s semimajor axis and also the planet’s mass. However, the planetary mass is actually a minimum mass because an orbital plane tilted out of our line of sight would require an even larger mass to produce the observed Doppler shift.
Discovering Planets from a Star’s Wobble: 51 Pegasi

39. Discovering Planets from a Star’s Wobble: 51 Pegasi
Figure 8-17: As a star wobbles, it moves toward us and away from us, with a varying radial velocity that is measured through the Doppler shift. The shape of the curve indicates the planet’s semimajor axis and also the planet’s mass. However, the planetary mass is actually a minimum mass because an orbital plane tilted out of our line of sight would require an even larger mass to produce the observed Doppler shift.
Discovering Planets from a Star’s Wobble: 51 Pegasi

40. A Selection of Extrasolar Planets
Figure 8-18: This figure summarizes what we know about planets orbiting a number of other stars. The star name is given at the left of each line. Each planet is shown at its average distance from its star (equal to the semimajor axis of its orbit), and some stars are found with multiple planets. The mass of each planet—actually a lower limit—is given as a multiple of Jupiter’s mass (MJ), equal to 318 Earth masses. Comparison with our own solar system (at the top of the figure) shows how closely many of these extrasolar planets orbit their stars—many planets about the size of Jupiter, orbit closer to their star than Mercury does to our Sun! Due to higher temperatures closer to the Sun, many of these exoplanets are called “hot Jupiters.” However, the radial velocity method detects large close-in planets most easily, and if all planets could be detected, the majority would be smaller and farther away. (Adapted from the California and Carnegie Planet Search)
A Selection of Extrasolar Planets

41. A Transiting Extrasolar Planet
Figure 8-19: If the orbit of an extrasolar planet is nearly edge-on to our line of sight, like the planet that orbits the star HD , we can learn about the planet’s (a) diameter, (b) atmospheric composition, and (c) surface temperature. (S. Seager and C. Reed, Sky and Telescope; H. Knutson, D. Charbonneau, R. W. Noyes (Harvard-Smithsonian CfA), T. M. Brown (HAO/NCAR), and R. L. Gilliland (STScI); A. Feild (STScI); NASA/JPL-Caltech/D. Charbonneau, Harvard-Smithsonian CfA)
A Transiting Extrasolar Planet

42. Earth-Size Exoplanets
Figure 8-20: Kepler-20 is a star with five known exoplanets. The exoplanets Kepler-20e and Kepler-20f are illustrated in comparison to Earth and Venus. Kepler-20e (at 0.05 AU) has a radius just 0.87 times smaller than Earth, while Kepler-20f (at 0.1 AU) is 1.03 times larger than Earth. However, these objects orbit extremely close to their star and are much too hot for liquid water on their surfaces. (Ames/JPL-Caltech/NASA)
Earth-Size Exoplanets

43. Exoplanets in the Habitable Zone
Figure 8-21: The exoplanet Kepler-22b lies inside the habitable zone and is compared to our solar system in this illustration. This planet is 2.4 times the size of Earth, which raises hopes for a hard surface with at least some liquid water. Its mass is not yet known, but it could be around 10 times the mass of Earth. Each star determines the size of its surrounding habitable zone, and the two zones in this image are similar because the star, Kepler-22, is Sun-like. (NASA/Ames/JPL-Caltech)
Exoplanets in the Habitable Zone

44. Microlensing Reveals an Extrasolar Planet
Figure 8-22: (a) A star with a planet drifts across the line of sight between a more distant star and a telescope on Earth. (b) The gravity of the closer star bends the light rays from the distant star, focusing the distant star’s light and making it appear brighter. (c) The gravity of the planet causes a second increase in the distant star’s brightness.
Microlensing Reveals an Extrasolar Planet

45. Figure 8-22a: A star with a planet drifts across the line of sight between a more distant star and a telescope on Earth.

46. Figure 8-22b: The gravity of the closer star bends the light rays from the distant star, focusing the distant star’s light and making it appear brighter.

47. Figure 8-22c: The gravity of the planet causes a second increase in the distant star’s brightness.

48. Problem 8.45: The image accompanying Question 45 shows a dark, opaque disk of material surrounding the young star IRAS The disk is edge-on to our line of sight, so it appears as a dark band running vertically across this image. The material to the left and right of this band is still falling onto the disk. (a) Make measurements on this image to determine the diameter of the disk in AU. Use the scale bar at the lower left of this image. (b) If the thickness of the disk is 50 AU, find its volume in cubic meters. (c) The total mass of the disk is perhaps 2×1028 kg (0.01 of the mass of the Sun). How many atoms are in the disk? Assume that the disk is all hydrogen. A single hydrogen atom has a mass of × 10–27 kg. (d) Find the number of atoms per cubic meter in the disk. Is the disk material thick or thin compared to the air that you breathe, which contains about 5.4 × 1025 atoms per cubic meter?

49. Key Ideas
The Nebular Hypothesis: The most successful model of the origin of the solar system is called the nebular hypothesis. According to this hypothesis, the solar system formed from a cloud of interstellar material called the solar nebula.
This occurred 4.54 billion years ago (as determined by radioactive dating).

50. Key Ideas
The Solar Nebula and Its Evolution: The chemical composition of the solar nebula, by mass, was 98% hydrogen and helium (elements that formed shortly after the beginning of the universe) and 2% heavier elements (produced much later in the centers of stars, and cast into space when the stars died).
The heavier elements were in the form of ice and dust particles.

51. Key Ideas
Formation of the Planets and Sun: The terrestrial planets, the Jovian planets, and the Sun followed different pathways to formation.
The four terrestrial planets formed through the accretion of dust particles into planetesimals, then into larger protoplanets.
Closer to the Sun – inside the snow line – water could not freeze into ice to help build larger planets. With rocks and metals making up only a fraction of mass throughout the nebula, the terrestrial planets made from these materials in the inner solar system are small.

52. Key Ideas
Jovian planets form beyond the snow line, where ices of water and methane contribute to make larger cores as these planets form. These larger cores can then attract copious amounts of hydrogen and helium gas, becoming larger Jovian planets
Gravitational interactions between the Jovian planets and a gaseous disk, and much later with a disk of planetesimals, leads to planetary migration. These migrations can explain the small size of Mars, the composition of the asteroid belt, the locations of the Jovian planets, aspects of the Kuiper belt and Oort cloud, and even account for a Late Heavy Bombardment.
The Sun formed by gravitational contraction of the center of the nebula. After about 108 years, temperatures at the protosun’s center became high enough to ignite nuclear reactions that convert hydrogen into helium, thus forming a true star.

53. Key Ideas
Extrasolar Planets: Astronomers have discovered planets orbiting other stars.
Many of these planets are detected by the “wobble” of the stars around which they orbit. The radial velocity method detects this wobble through Doppler shifts. The astrometric method directly observes a star’s change in position as it wobbles.
Many extrasolar planets have been discovered by the transit method and a lesser number by microlensing, and direct imaging.
Most of the extrasolar planets discovered to date are quite massive and have orbits that are very different from planets in our solar system.